High-strength cold-rolled steel sheet excellent in coating film adhesion

ABSTRACT

A cold-rolled steel sheet of DP (Dual Phase) type with a specific composition meets the requirements: 
     (I) In the surface of the steel sheet, there exist Si—Mn complex oxides no larger than 5 μm in diameter of the equivalent circle as many as 10 or more per 100 μm 2  and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area, and/or (II) The cross section near the surface of the steel sheet does not show cracks with a width no larger than 3 μm and a depth no smaller than 5 μm in arbitrary ten fields of observation under an SEM with a magnification of 2000. A high-strength cold-rolled steel sheet excellent in coating film adhesion and having a tensile strength no lower than 550 MPa is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength cold-rolled steel sheetexcellent in coating film adhesion, and more particularly, to acold-rolled steel sheet which has a tensile strength no lower than 550MPa and is suitable for use as a steel sheet for automobile parts onaccount of its excellent coating film adhesion.

2. Description of the Related Art

There is a growing demand for high-strength steel products necessary forautomobiles with less fuel consumption and lower weight than before.This trend is prevailing also in the field of cold-rolled steel sheet.On the other hand, cold-rolled steel sheets are required to havesufficient ductility (such as elongation) because they are formed intoautomotive parts by pressing. Increase in strength can be effectivelyachieved by incorporation with alloying elements; however, theyadversely affect ductility as their amount increases.

Of these alloying elements, Si is less influential in reducing ductilityand is effective in increasing strength while retaining ductility.However, with an increased Si content, the resulting steel sheet is poorin chemical treatability and hence in coating film adhesion.Consequently, it was necessary to reduce Si content in the case wherechemical treatability is important. Moreover, excess Si forms anSi-containing intergranular oxide on the surface of steel sheet, therebycausing cracks to occur and aggravating coating film adhesion.

One way to reconcile mechanical properties and chemical treatability isby cladding a steel sheet of high Si content with a layer of low Sicontent. Such a cladding layer contributes to chemical treatabilitywithout adverse effect on the mechanical properties of the steel sheet.(See Japanese Patent Laid-open No. Hei-5-787452) And the steel sheet ofhigh Si content ensures sufficient mechanical properties. Unfortunately,cladding needs a complex process which leads to an increased productioncost.

There is a conventional technology of adding a special alloying element,such as Ni and Cu, which prevents Si (detrimental to chemical treatment)from concentrating in the surface of a steel sheet. (See Japanese PatentNo. 2951480 and Japanese Patent No. 3266328) This technology suffers adisadvantage of requiring expensive Ni or Cu, which leads to an increasein production cost.

The conventional technology mentioned above is concerned with so-calledIF (Interstitial Free) steel. IF steel is limited in carbon content (nomore than 0.005%) and has its texture controlled by a specificrecrystallization temperature, so that it is improved in deepdrawability. However, IF steel with a very low carbon content will notachieve the high strength intended by the present invention.

There is a technology of ensuring chemical treatability by causing NbCto separate out and function as a site for nucleation of zinc phosphatecrystals. (See Japanese Patent No. 2003-3049147) This technology is alsodesigned to improve deep drawability by keeping the carbon content low(no more than 0.02%) for texture control. The steel of this technologyhas a slightly higher carbon content than the above-mentioned IF steel,but is still unsatisfactory in strength. Japanese Patent No. 3049147discloses two inventions which respectively achieve a strength of 539MPa (55 kgf/mm²) and 588 MPa (60 kgf/mm²) which is in excess of 550 MPa.This strength has been realized by increasing the content of P or Mo.Unfortunately, these elements are detrimental to weldability.

There has been proposed a retained austenite-containing steel sheetwhich has good chemical treatability owing to the controlled ratio ofSiO₂/Mn₂SiO₄ in the surface layer. (See Japanese Patent Laid-open No.2003-201538) To obtain this steel sheet, it is necessary to controloxides in the surface layer, to perform pickling or brushing on thesurface after continuous annealing, thereby removing Si oxides andcontrolling the Si/Fe ratio, and to keep the dew point above −30° C. atthe temperature below the Ac₁ transformation point, thereby limiting theamount of Si oxides to be formed.

Unfortunately, pickling and brushing increase the number ofmanufacturing steps, which leads to a higher production cost. Inaddition, the control of dew point, which is accomplished in acontinuous annealing furnace, is not very effective so long as Examplesshow in the document. According to data in the document, the ratio ofSiO₂/Mn₂SiO₄ in the surface layer is about 1.0. This value suggests thatSiO₂, which prevents the formation of film crystals due to chemicaltreatment, occurs as much as Mn₂SiO₄. Judging from these results, thedisclosed technology will not sufficiently improve chemicaltreatability.

Moreover, the retained austenite-containing steel sheet mentioned abovecontains such alloying elements as C, Si, Mn, and Al in large amounts soas to secure retained austenite. Therefore, it is poor in weldability.

There has been proposed another technology of improving chemicaltreatability, which is intended to keep below 1 the Si/Mn ratio inoxides determined by surface analysis with XPS (X-ray photoelectronspectroscopy). (See Japanese Patent Laid-open No. Hei-4-276060)

An example of steel having an Si/Mn ratio lower than 1 is mild steelnearly free of Si, which is known to have good chemical treatability.However, a certain amount of Si is necessary for steel to have both highstrength and good ductility, and hence there is a limit of reducing theSi content to keep the Si/Mn ratio below 1. Further, it turned out thata steel sheet does not always exhibit good chemical treatability eventhough it has an Si/Mn ratio lower than 1, for a certain Si content andan adequately controlled Mn content.

SUMMARY OF THE INVENTION

The present invention was completed in view of the foregoing. It is anobject of the present invention to provide a cold-rolled steel sheetcharacterized by a tensile strength no lower than 500 MPa and excellentcoating film adhesion and weldability.

The present invention is directed to a high-strength cold-rolled steelsheet excellent in coating film adhesion, which is a DP (Dual Phase)steel sheet of ferrite-tempered martensite type containing no more than1 mass % of C (excluding 0 mass %), 0.05 to 2 mass % of Si, and 1 to 5mass % of Mn, having a tensile strength no lower than 550 MPa,satisfying the equation (1) below, and being characterized by itssurface in which there exist Si—Mn complex oxides no larger than 5 μm indiameter of the equivalent circle as many as 10 or more per 100 μm² andthe coverage of oxides composed mainly of Si on the surface of steelsheet is no more than 10% of surface area (requirement (I)). Theequivalent circle means the circle of the same area as the Si—Mn complexoxide. (This steel sheet will be referred to as “Steel sheet 1 of thepresent invention” hereinafter.)[Si]/[Mn]≦0.4  (1)where [Si] denotes an Si content (in mass %) and [Mn] denotes an Mncontent (in mass %).

The term “oxides composed mainly of Si” mentioned above means thoseoxides in which Si (as one of the constituents excluding oxygen)accounts for no less than 70% in atomic ratio. Such oxides areconsidered to be amorphous according to the result of analysis.

The ratio of the surface area of steel sheet which is covered by theoxides composed mainly of Si was obtained by observation under a TEM(Transmission Electron Microscope), quantitative analysis and mapping ofSi, O, Mn, and Fe by EDX (Energy Dispersive X-ray), and image analysisof these data. Observation under a TEM was accomplished by using anextraction replica, which is explained in Examples given later.Observation under a TEM for an extraction replica may be replaced bysurface mapping for Si, O, Mn, and Fe by AES (Auger ElectronSpectroscopy) at a magnification of 2000 to 5000, and the resulting datamay be used for image analysis.

The present invention is directed also to a high-strength cold-rolledsteel sheet excellent in coating film adhesion, which is a DP (DualPhase) steel sheet of ferrite-tempered martensite type containing nomore than 1 mass % of C (excluding 0 mass %), no more than 2 mass % ofSi (excluding 0 mass %), and 1 to 5 mass % of Mn, having a tensilestrength no lower than 550 MPa, and being characterized by its surfacewhose cross section does not show cracks with a width no larger than 3μm and a depth no smaller than 5 μm in arbitrary ten fields ofobservation under an SEM (Scanning Electron Microscope) with amagnification of 2000 (requirement (II)). (This steel sheet will bereferred to as “Steel sheet 2 of the present invention” hereinafter.)

The width and depth of cracks are shown in FIG. 1 (which is a schematicsectional view of the steel sheet). They are found by observing thevicinity of the surface of the steel sheet under an SEM with amagnification of 2000 (Model S-4500 of Hitachi Ltd.).

The present invention is directed also to a high-strength cold-rolledsteel sheet excellent in coating film adhesion, which is a DP (DualPhase) steel sheet of ferrite-tempered martensite type containing nomore than 1 mass % of C (excluding 0 mass %), 0.05 to 2 mass % of Si,and 1 to 5 mass % of Mn, having a tensile strength no lower than 550MPa, satisfying the equation (1) above, and meeting the above-mentionedrequirements (I) and (II). (This steel sheet will be referred to as“Steel sheet 3 of the present invention” hereinafter.)

The steel sheets of the present invention should preferably have acomposition specified by the equations (2) and (3) below as anadditional requirement, so that they exhibit good weldability.[P]+3[S]+1.54[C]<0.25  (2)[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3)where [C], [Si], [Mn], [P], and [S] denote the content (in mass %) ofthese elements.

The steel sheet according to the present invention has a high strengthin excess of 550 MPa, exhibits good chemical treatability, and/or goodcoating film adhesion owing to controlled fine cracks, and provides goodweldability. It is suitable for automotive parts. It can be producedwithout cladding or expensive elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing cracks in the cross section of thesteel sheet.

FIG. 2 is a diagram illustrating (part of) one manufacturing process inExamples.

FIG. 3 is a diagram illustrating (part of) another manufacturing processin Examples.

FIG. 4 is an electron micrograph (TEM) of sample in experiment No. 1 inExamples. (Extraction replica, ×15000)

FIG. 5 is an electron micrograph (TEM) of sample in experiment No. 29 inExamples. (Extraction replica, ×15000)

FIG. 6 is an electron micrograph (TEM) of sample in experiment No. 34 inExamples. (Extraction replica, ×15000)

FIG. 7 is an electron micrograph (SEM) showing the cross section nearthe surface of the steel sheet in experiment No. 1 in Examples.

FIG. 8 is an electron micrograph (SEM) showing the cross section nearthe surface of the steel sheet in experiment No. 29 in Examples.

FIG. 9 is an electron micrograph (SEM) showing the cross section nearthe surface of the steel sheet in experiment No. 34 in Examples.

FIG. 10 is an electron micrograph (SEM) showing the surface of the steelsheet (after chemical treatment) in experiment No. 1 in Examples.

FIG. 11 is an electron micrograph (SEM) showing the surface of the steelsheet (after chemical treatment) in experiment No. 29 in Examples.

FIG. 12 is an electron micrograph (SEM) showing the surface of the steelsheet (after chemical treatment) in experiment No. 34 in Examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The results of investigation carried out to obtain a steel sheetexcellent in coating film adhesion revealed that the object is achievedif the following requirements (I) and/or (II) are met. This finding ledto the present invention. The steel sheet that meets these requirementsand has a high strength (in excess of 550 MPa) and good ductility can beproduced in specific compositions under specific manufacturingconditions as mentioned later.

(I) In the surface of steel sheet:

-   -   (i) there should exist Si—Mn complex oxides no larger than 5 μm        in diameter of the equivalent circle as many as 10 or more per        100 μm², and    -   (ii) the coverage of oxides composed mainly of Si on the surface        of steel sheet should be no more than 10% of the surface area.        (“Mainly” means that Si accounts for no less than 70% (in atomic        ratio) in the constituents of oxides other than oxygen.)

(II) The cross section of the surface of steel sheet should not showcracks with a width no larger than 3 μm and a depth no smaller than 5 μmin arbitrary ten fields of observation under an SEM with a magnificationof 2000.

The above-mentioned requirements (I) and (II) were established for thefollowing reasons.

Requirement that in the surface of steel sheet, there should exist Si—Mncomplex oxides no larger than 5 μm in diameter of the equivalent circleas many as 10 or more per 100 μm².

The present inventors have carried out a series of researches to obtaina high-strength steel sheet excellent in coating film adhesion andproposed a technique for improving the chemical treatability of a steelsheet containing Si in a comparatively large amount. (See JapanesePatent Application No. 2003-106152.) This technique is intended toimprove chemical treatability by finely dispersing amorphous Si oxidesdetrimental to chemical treatability while controlling the annealingatmosphere. However, major oxides that occur when the Si content isrelatively low (Si content: 0.05 to 2% as defined in the presentinvention) are Si—Mn complex oxides rather than amorphous Si oxides. Itis considered that these complex oxides are also detrimental to coatingfilm adhesion. With this in mind, the present inventors searched for thepossibility of positively using Si—Mn complex oxides for improvement ofchemical treatability.

As the result, it turned out that chemical treatability improves ifSi—Mn complex oxides are finely dispersed into iron oxides formed in thesurface layer of steel sheet, so as to form the “inhomogeneous field ofoxide interface” which functions as the nucleating site for zincphosphate crystals (as mentioned later). It is not yet elucidated whythe Si—Mn complex oxides specified in the present invention function asthe nucleating site for zinc phosphate crystals. A probable reason is asfollows.

It is known that zinc phosphate crystals tend to form during chemicaltreatment in the “electrochemical inhomogeneous field” originating fromthe grain boundary or the periphery of the Ti colloid which has beenattached to the surface of steel sheet at the time of surfacepreparation. It is considered that the Si—Mn complex oxides specified inthe present invention also create the electrochemical inhomogeneousfield around them, thereby helping zinc phosphate crystals to stickeasily at the time of chemical treatment, which leads to improvedchemical treatability.

It is considered that zinc phosphate crystals after chemical treatmentshould preferably be no larger than several micrometers from thestandpoint of coating film adhesion. Consequently, it is also consideredthat the electrochemical inhomogeneous field mentioned above shouldpreferably be of the same size. For this reason, the present inventionspecifies that there should exist Si—Mn complex oxides no larger than 5μm in diameter of the equivalent circle as many as 10 or more per 100μm² (or 1 per 10 μm² or more on average), with the average distancebetween particles of complex oxides being several micrometers. Thiscondition is necessary for easy formation of the electrochemicalinhomogeneous field of the above-specified size.

The number of particles of Si—Mn complex oxides should preferably be 50or more per 100 μm², more preferably 100 per or more 100 μm², and mostdesirably 150 or more per 100 μm², because the electrochemicalinhomogeneous field does not necessarily occur in every particle of theSi—Mn complex oxides which are present. An example of the Si—Mn complexoxides is Mn₂SiO₄. It is considered that about 50 nm is the maximumobservable size of Si—Mn complex oxides.

Requirement that the coverage of oxides composed mainly of Si on thesurface of steel sheet should be no more than 10% of surface area.

The Si—Mn complex oxides functioning as nucleating sites for zincphosphate crystals will not contribute to good chemical treatability ifthere exist other substances detrimental to chemical treatment. Hence,the resulting steel sheet will be poor in coating film adhesion.

If oxides composed mainly of Si are present on the surface of steelsheet, zinc phosphate crystals do not form on them, which leads toconsiderably poor chemical treatability. Consequently, the presentinvention requires that the coverage of oxides composed mainly of Si onthe surface of steel sheet should be no more than 10% of surface area.

The present inventors had previously proposed a technique of improvingchemical treatability by finely dispersing oxides composed mainly of Si,as mentioned above. However, it turned that that the presence of oxidesshould be minimized in the present invention which is intended toutilize the action of Si—Mn complex oxides as mentioned above.Therefore, the coverage of oxides composed mainly of Si on the surfaceof steel sheet should preferably be no more than 5% of surface area,most desirably 0% of surface area.

Requirement that the cross section of the surface layer of the steelsheet does not show cracks with a width no larger than 3 μm and a depthno smaller than 5 μm in arbitrary ten fields of observation under an SEMwith a magnification of 2000.

Sharp cracks present on the surface of the steel sheet prevent zincphosphate crystals from sticking to them at the time of chemicaltreatment. As the result, corrosion readily proceeds there, aggravatingcoating film adhesion. For this plausible reason, it is important tominimize the occurrence of sharp cracks in order to improve coating filmadhesion.

The present inventors had previously proposed a technique of improvingcoating film adhesion by restricting to 10 μm or less the depth oflinear oxides (narrower than 30 nm) composed of Si and oxygen. Thistechnique is based on the assumption that continuous annealing will notbe followed by pickling. However, in common practice, continuousannealing is followed by pickling, and pickling removes linear oxides,thereby causing cracks to occur.

How the depth of cracks relates with linear oxides is not yetquantitatively elucidated. It is considered that linear oxides dissolvein acid or mechanically drop off, thereby giving rise to cracks. It isalso considered that such cracks are deeper than the size of linearoxides because they dissolve further in acid even after linear oxideshave been removed.

With the foregoing in mind, the present inventors conceived that itwould be possible to improve coating film adhesion more by controllingcracks than by regulating the depth of linear oxides (as in thetechnology they had previously proposed) and they investigated the shapeof cracks to be controlled. As the result, it was found that zincphosphate crystals hardly stick to cracks having a width approximatelyequal to or smaller than their particle diameter. This holds trueparticularly for cracks deeper than 5 μm. Thus, according to the presentinvention, cracks to be controlled are limited to those which arenarrower than 3 μm and deeper than 5 μm.

Based on the foregoing is established the requirement that the crosssection of the surface layer of the steel sheet should not show theabove-specified cracks in arbitrary ten fields of observation under anSEM with a magnification of 2000.

The steel sheet according to the present invention is required to havethe following chemical composition so that it has controlled cracks forefficient deposition of the above-mentioned oxides and it exhibits thecharacteristic properties of high-strength steel sheet.[Si]/[Mn]≦0.4  (1)where [Si] denotes an Si content (in mass %) and [Mn] denotes an Mncontent (in mass %).

Since oxides composed mainly of Si adversely affect chemicaltreatability, it is more desirable to suppress them as much as possiblerather than finely dispersing them. The object of suppressing suchoxides can be achieved if the [Si]/[Mn] ratio in the chemicalcomposition is no larger than 0.4, preferably no larger than 0.3.

-   C: no larger than 1 mass % (excluding 0 mass %)

Carbon is essential for strength. The minimum carbon content is 0.05mass %. An excess carbon content aggravates weldability. Therefore, thecarbon content should be no larger than 1 mass %, preferably no largerthan 0.23 mass %, and more preferably no larger than 0.15 mass %.

-   Si: 0.05 to 2 mass % (for steel sheets 1 and 3)-   Si: no larger than 2 mass % (excluding 0 mass %) (for steel sheet 2)

Since Si increases strength without decreasing ductility, it may becontained in the steel sheet. A certain amount of Si is necessary forthe Si—Mn complex oxides with a diameter of the equivalent circle nolarger than 5 μm to form as much as specified by the requirement (I)mentioned above. A minimum amount of Si for this purpose is 0.05 mass %.An adequate amount should be no less than 0.15 mass %, preferably noless than 0.3 mass %, and more preferably no less than 0.5 mass %. Si inan excess amount brings about solid solution hardening more thannecessary, which leads to an increased rolling load. Therefore, thecontent of Si should be no larger than 2 mass %, preferably no largerthan 1.5 mass %.

-   Mn: 1 to 5 mass %

Mn is also essential for strength; however, excess Mn is detrimental toductility. An adequate content of Mn should be no less than 1 mass %,preferably no less than 2 mass %, and no more than 5 mass %, preferablyno more than 3.5 mass %.

The steel sheet according to the present invention should contain theabove-mentioned elements, with the remainder being substantially iron.It may contain inevitable impurities, such as Al no more than 1 mass %,N no more than 0.01 mass %, and O no more than 0.01 mass %, originatingfrom raw materials or incorporated depending on production conditions.It may be positively incorporated with additional elements, such as Cr,Mo, Ni, Ti, Nb, V, P, and B, in an amount not harmful to the effect ofthe present invention.

The amount of these additional elements to strengthen the steel sheet isspecified as follows.

-   Cr: 0.1 to 1 mass %-   Mo: 0.1 to 1 mass %-   Ni: 0.1 to 1 mass %-   Ti: 0.005 to 0.1 mass %-   Nb: 0.005 to 0.1 mass %-   V: 0.0005 to 0.01 mass %-   P: 0.005 to 0.1 mass %-   B: 0.0003 to 0.01 mass %    These additional elements will aggravate ductility and weldability    when added in an excess amount.-   [P]+3[S]+1.54[C]<0.25  (2)    [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3)    where [C], [Si], [Mn], [P], and [S] denote the content (in mass %)    of these elements.

The left side of each of the equations (2) and (3) above is known as theparameter to evaluate spot weldability. {Tanaka et al., Nippon KoukanGihou, No. 105 (1984); Heuschkel, J.: Weld J26 (10), P560 S(1947)} Themore the parameter increases, the more the weldability decreases. It wasfound in the present invention that spot weldability decreases when theleft hand in (2) and (3) exceeds 0.25 and 0.34, respectively.

The present invention covers a steel sheet having a strength no lowerthan 550 MPa (preferably no lower than 750 MPa, more preferably no lowerthan 900 MPa). The steel sheet should contain C, Si, and Mn (andoptionally P) in an adequate amount as specified below according tostrength and weldability desired.

For tensile strength from 550 to 650 MPa.[P]+3[S]+1.54[C]<0.14[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.21

For tensile strength from 650 (exclusive) to 750 MPa.[P]+3[S]+1.54[C]<0.18[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.27

For tensile strength from 750 (exclusive) to 1050 MPa.[P]+3[S]+1.54[C]<0.22[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.30

For tensile strength in excess of 1050 MPa.[P]+3[S]+1.54[C]<0.25[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34

The present invention covers a DP (Dual Phase) steel sheet offerrite-tempered martensite type. The steel may be composed solely offerrite and tempered martensite, or it may additionally containpearlite, bainite, and retained austenite in an amount not harmful tothe effect of the present invention. They inevitably remain in themanufacturing process, bur they should be as little as possible.

For the steel sheet to have good chemical treatability, the shape of theoxides that separate out on the surface thereof should be controlledaccording to the requirement (I) mentioned above. This object isachieved not only by the controlled steel composition as mentioned abovebut also by pickling (that follows hot rolling) with hydrochloric acid(1 to 18 mass %) at 70 to 90° C. for about 40 seconds or more(preferably 60 seconds or more) and continuous annealing in anatmosphere with a dew point no higher than −40° C., preferably no higherthan −45° C. Incidentally, in the case of pickling with several acidbaths for intermittent dipping, the total dipping time should be 40seconds at the minimum. Thus, in order to control the formation of theoxides so as to satisfy the requirement (I), the hot rolled steel sheetneed undergo pickling.

For the steel sheet to be free of cracks as provided in the requirement(II) mentioned above, the manufacturing process is specified as follows:

The winding temperature for hot rolling should be no higher than 500°C., preferably no higher than 480° C.; The hot rolled steel sheet shouldbe dipped in hydrochloric acid (1 to 18 mass %) at 70 to 90° C. for 40seconds or more, preferably 60 seconds or more; Continuous annealingshould be performed in an atmosphere with a dew point no higher than−40° C., preferably no higher than −45° C.; The hardening starttemperature at the time of annealing (which may be referred to as “slowcooling end temperature”) should be no higher than 550° C., preferablyfrom 400 to 450° C. Thus, in order to restrict the formation of thecracks and to satisfy the requirement (II), the production conditionsshould be such that generation of grain boundary oxides which can becomestart points for the cracks is restrained.

The present invention does not specify other manufacturing conditionsthan mentioned above. Thus the steel sheet may be produced in the usualway by melting, casting, and hot rolling. Although the manufacturingprocess in Examples that follow involves pickling that followscontinuous annealing, pickling is not mandatory in the presentinvention.

EXAMPLES

The invention will be described in more detail with reference to thefollowing examples, which are not intended to restrict the scopethereof, and various changes and modifications may be made in theinvention without departing from the spirit and scope thereof.

In each example, a steel with the chemical composition shown in Table 1was prepared by melting, and the resulting steel was cast into a slab,which underwent hot rolling, followed by pickling. Winding and picklingwere performed under the conditions shown in Tables 2 and 3. Picklinginvolved an aqueous solution of hydrochloric acid (1 to 18 mass %) at 70to 90° C. Pickling was followed by cold rolling, which gave a 1.4 mmthick steel sheet.

The steel sheet underwent continuous annealing by either of theprocesses shown in FIGS. 2 and 3. The process shown in FIG. 2 involvescooling with water quenching (WQ) that follows soaking and slow cooling.The process shown in FIG. 3 involves cooling with mist, gas blowing(GJ), or water-cooled roll quenching (RQ).

The heating temperature, slow cooling end temperature, and temperingtemperature shown in Tables 2 and 3 correspond to those shown in FIGS. 2and 3. The dew point is that of the atmosphere in the continuousannealing furnace. After cooling, the steel sheet underwent tempering.In the process shown FIG. 2, pickling was carried out before and/orafter tempering.

The thus obtained steel sheet was examined for mechanical properties andcoating film adhesion. All of the steel sheet samples were found to becomposed mainly of ferrite and tempered martensite.

Mechanical properties were determined by measuring the tensile strength(TS), total elongation (El), and yield ratio (YP) of specimens(conforming to JIS No. 5) taken from the steel sheet. A discoid specimenmeasuring 100 mm in diameter and 1.4 mm thick was tested forstretch-flanging performance. The test method consists of punching ahole (10 mm in diameter) in the specimen and expanding the hole by a 60°conical punch, with the burr upward. The bore expanding ratio (λ) wasmeasured when the conical punch passed through with cracking. (accordingto JFST 1001 provided by The Japan Iron and Steel Federation).

For evaluation of coating film adhesion, samples were examined forchemical treatability and the presence of cracks in the followingmanner. After chemical treatment, the surface of the treated steel sheetwas observed under an SEM with a magnification of 1000 to confirm thepresence of zinc phosphate crystals in ten fields of observation. Theresult was rated according to the following criterion.

-   ◯: Zinc phosphate crystals are present uniformly in all of ten    fields of observation.-   x: There is at least one field of observation in which zinc    phosphate crystals are not present.

Method for Chemical Treatment

-   -   Chemical treating solution: “Parbond L3020” from Nihon        Parkerizing Co., Ltd.    -   Process of chemical treatment: degreasing→water washing→surface        conditioning→chemical treatment        Method of Counting the Number of Si—Mn Oxide Particles

First, an extraction replica is prepared from the surface of the steelsheet. Then, it is observed under a TEM (Model H-800 of Hitachi, Ltd.)with a magnification of 15000. An average number of particles (per 100μm²) is counted in arbitrary 20 fields of observation.

The ratio of the surface area of steel sheet which is covered by theoxides composed mainly of Si was obtained by observation of a sampleunder a TEM and ensuing image analysis. The sample was prepared by theextraction replica method consisting of four steps (a) to (d) asexplained in the following.

(a) Vacuum deposition of carbon on the surface of steel sheet.

(b) Cross-cutting (2 to 3 mm square each) of the sample surface.

(c) Corrosion with an etching solution composed of 10% acetylacetone and90% methanol. This corrosion brings carbon into relief.

(d) Storing the sample in alcohol for observation.

The treated sample was photographed in ten fields of observation (eachmeasuring 13 by 11 cm) through a TEM with a magnification of 15000. Theresulting electron micrograph was examined to measure the area coveredby oxides composed mainly of Si (or oxides in which Si as theconstituents excluding oxygen accounts for no less than 70% in atomicratio). In this way there was obtained the coverage of oxides composedmainly of Si.

The presence of cracks (width: no larger than 3 μm, depth: no smallerthan 5 μm) was examined by observing the cross section of the surfacelayer of the steel sheet in ten fields of observation (each measuring 13by 11 cm) under an SEM (Model S-4500 of Hitachi Ltd.) with amagnification of 2000. TABLE 1 Composition (mass %) Type C + Si/3O + ofB N O P + 3S + Mn/20 + steel C Si Mn P S Al Cr Mo Ti Nb V (ppm) (ppm)(ppm) Si/Mn 1.54C 2P + 4S 1 0.08 0.69 2.45 0.001 0.001 0.029 — 0.2 — — —— 18 21 0.282 0.13 0.23 2 0.08 1.01 3.11 0.008 0.003 0.033 — — — — — — 915 0.325 0.14 0.30 3 0.08 0.21 2.91 0.008 0.003 0.014 — — — — — — 18 310.072 0.14 0.26 4 0.08 0.59 2.99 0.007 0.003 0.015 — — — — — — 22 360.197 0.14 0.27 5 0.08 1.01 3.04 0.007 0.002 0.015 — — — — — — 23 320.332 0.13 0.29 6 0.10 0.47 1.61 0.011 0.002 0.024 — 0.19 — — — — 30 280.292 0.17 0.23 7 0.10 0.66 2.10 0.013 0.003 0.026 — 0.18 — — — — 28 230.314 0.18 0.27 8 0.13 0.19 2.68 0.010 0.011 0.019 — — 0.050 — — — 34 240.071 0.22 0.30 9 0.08 0.73 2.39 0.006 0.002 0.047 — 0.2  — 0.001 0.0035 15 20 0.305 0.13 0.24 10 0.08 0.96 2.95 0.008 0.003 0.038 — — — 0.0030.005 6 20 23 0.325 0.14 0.29 11 0.05 1.02 2.98 0.003 0.005 0.066 — — —— — — 12 16 0.324 0.10 0.28 12 0.05 0.98 2.92 0.003 0.006 0.062 0.20 — —— — — 15 15 0.336 0.10 0.27 13 0.05 0.98 2.87 0.002 0.007 0.060 0.40 — —— — — 8 11 0.341 0.11 0.27 14 0.05 1.00 2.87 0.003 0.007 0.066 0.59 — —— — — 13 12 0.341 0.11 0.28 15 0.05 1.00 3.05 0.011 0.007 0.064 0.80 — —— — — 11 10 0.328 0.11 0.29 16 0.05 0.99 3.09 0.010 0.005 0.043 — 0.19 —— — — 24 22 0.320 0.11 0.28 17 0.05 0.99 3.08 0.010 0.005 0.048 — 0.47 —— — — 22 22 0.321 0.11 0.28 18 0.05 0.99 2.91 0.010 0.007 0.049 0.200.19 — — — — 22 23 0.340 0.11 0.28 19 0.05 0.98 2.85 0.010 0.006 0.0440.21 0.47 — — — — 21 21 0.343 0.11 0.27 20 0.11 1.01 2.93 0.011 0.0070.092 — — — — — — 33 8 0.345 0.11 0.27 21 0.07 0.19 2.00 0.004 0.0060.039 — — — 0.020 — — 37 30 0.095 0.13 0.22 22 0.05 0.48 1.98 0.0040.005 0.033 — — — 0.022 — — 41 31 0.242 0.13 0.22 23 0.08 0.49 1.910.004 0.005 0.035 — — 0.010 0.020 — — 39 27 0.257 0.14 0.25 24 0.07 0.481.94 0.004 0.006 0.036 — — — 0.035 — — 42 27 0.247 0.14 0.23 25 0.191.66 2.03 0.002 0.001 0.029 — — — — — — 22 29 0.818 0.30 0.35 26 0.100.82 1.86 0.010 0.002 0.067 — — 0.066 — — — 27 13 0.441 0.17 0.25 270.14 0.24 1.87 0.011 0.007 0.055 — — — — — — 30 17 0.128 0.25 0.29 280.11 1.05 2.31 0.004 0.002 0.037 — — 0.042 — — — 28 26 0.455 0.18 0.2829 0.10 0.01 2.57 0.005 0.003 0.038 0.10 0.09 0.008 0.008 — — 25 190.004 0.17 0.25 30 0.10 1.96 2.49 0.004 0.003 0.040 0.09 0.10 0.0090.010 — — 23 20 0.787 0.17 0.31 31 0.10 0.65 1.55 0.004 0.003 0.039 0.100.09 0.007 0.008 — — 20 18 0.419 0.17 0.22 32 0.10 0.63 3.53 0.006 0.0030.040 0.11 0.09 0.009 0.009 — — 26 18 0.178 0.17 0.32 33 0.16 0.63 2.590.011 0.005 0.057 0.021 — — — 0.007 — 17 20 0.243 0.27 0.35

TABLE 2 Manufacturing conditions Surface oxides Slow Si—Mn Ex- Wind-Pick- Cooling Tem- *1 peri- Steel ing ling Heating End pering DewMechanical properties oxides Si *2 Film adhesion ment Kind Temp. TimeTemp. Temp. Cooling Temp. Point TS EL YP λ (numb- oxides Chemical No.No. (° C.) (sec) (° C.) (° C.) Method (° C.) (° C.) (MPa) (%) (MPa) (%)er) (%) treatability Cracks 1 1 450 50 850 500 WQ 180 −50 985 17 639 4220 0 ◯ none 2 2 450 50 850 440 WQ 200 −50 1007 18 697 52 41 0 ◯ none 3 3500 50 830 500 WQ 200 −40 849 18 560 33 16 0 ◯ none 4 4 500 50 830 500WQ 200 −40 984 16 679 36 26 0 ◯ none 5 5 500 50 830 500 WQ 200 −40 101716 651 28 43 3 ◯ none 6 6 500 50 830 450 WQ 180 −40 633 30 396 55 12 2 ◯none 7 7 500 50 830 450 WQ 180 −40 802 23 529 42 24 3 ◯ none 8 8 500 50830 450 WQ 250 −40 1203 12 866 31 13 0 ◯ none 9 9 500 50 830 450 WQ 180−40 1004 17 664 43 18 3 ◯ none 10 10 500 50 830 500 WQ 200 −40 998 17637 34 39 2 ◯ none 11 11 450 60 870 400 WQ 200 −50 835 22 502 35 35 2 ◯none 12 12 450 60 870 420 WQ 200 −50 871 20 545 33 34 2 ◯ none 13 13 45060 870 440 WQ 200 −50 902 19 599 30 35 4 ◯ none 14 14 450 60 870 460 WQ200 −50 997 16 702 35 37 5 ◯ none 15 15 450 60 870 400 WQ 200 −50 100813 734 38 36 2 ◯ none 16 16 450 60 870 380 WQ 250 −50 981 16 667 45 35 3◯ none 17 17 450 60 870 380 WQ 250 −50 1012 12 784 40 35 3 ◯ none 18 18450 60 870 380 WQ 180 −50 987 15 658 35 31 2 ◯ none 19 19 450 60 870 380WQ 180 −50 1003 13 772 41 29 5 ◯ none 20 20 450 60 870 450 WQ 400 −501052 14 910 35 38 4 ◯ none 21 21 400 40 850 520 WQ 400 −45 617 27 523 4913 0 ◯ none 22 22 400 40 850 520 WQ 400 −45 608 28 499 58 18 0 ◯ none 2323 400 40 850 520 WQ 400 −45 708 25 554 43 19 0 ◯ none*1 Number of particles of Si-Mn complex oxides (smaller than 1.5 μm) per100 μm².*2 Ratio of the surface area of steel sheet which is covered with oxidescomposed mainly of Si.

TABLE 3 Manufacturing conditions Surface oxides Slow Si—Mn Ex- Wind-Pick- Cooling Tem- *1 peri- Steel ing ling Heating End pering DewMechanical properties oxides Si *2 Film adhesion ment Kind Temp. TimeTemp. Temp. Cooling Temp. Point TS EL YP λ (numb- oxides Chemical No.No. (° C.) (sec) (° C.) (° C.) Method (° C.) (° C.) (MPa) (%) (MPa) (%)er) (%) treatability Cracks 24 24 400 40 850 520 WQ 400 −45 611 26 52745 20 0 ◯ none 25 1 450 50 850 500 RQ 180 −45 948 18 685 44 18 0 ◯ none26 1 450 50 850 500 mist 180 −40 953 17 673 40 25 0 ◯ none 27 1 450 50850 500 GJ 180 −40 902 19 667 48 22 0 ◯ none 28 33 480 50 850 450 WQ 200−40 1285 13 882 25 21 0 ◯ none 29 25 550 50 880 630 WQ 230 −40 1006 19649 26 8 55 X yes 30 26 600 50 880 630 WQ 400 −40 1215 11 972 28 9 25 Xyes 31 28 600 40 850 650 mist 250 −40 990 17 599 18 8 33 X yes 32 27 65050 850 650 GJ 500 −40 608 26 394 47 13 0 ◯ yes 33 1 450 50 850 500 WQ180 0 972 16 659 38 5 5 X yes 34 29 480 50 850 500 WQ 200 −45 1056 9 74427 0 0 X none 35 30 480 50 850 500 WQ 200 −45 1227 11 937 25 7 40 X none36 31 480 50 850 500 WQ 200 −45 735 20 558 42 9 20 X none 37 32 480 50850 500 WQ 200 −45 1327 8 1002 19 30 0 ◯ none 38 5 700 50 830 500 WQ 200−40 992 16 631 30 45 8 ◯ yes 39 5 500 5 830 500 WQ 200 −40 1021 15 66027 9 12 X yes 40 5 500 50 830 700 WQ 200 −40 1217 11 1010 48 48 5 ◯ yes41 5 500 50 830 500 WQ 200 −10 1007 16 654 33 8 15 X yes*1 Number of particles of Si-Mn complex oxides (smaller than 1.5 μm) per100 μm².*2 Ratio of the surface area of steel sheet which is covered with oxidescomposed mainly of Si.

The results shown in Tables 1 to 3 are discussed in the following. (No.means Experiment No.)

Samples in Nos. 32, 38, and 40 meet the requirement for the steel sheet1 of the present invention and hence they are excellent in chemicaltreatability and coating film adhesion. The results suggest that it isnecessary to control the winding temperature and slow cooling endtemperature for the steel sheet to have good coating film adhesion, withcracks property controlled.

Samples in Nos. 34 to 36 meet the requirement for the steel sheet 2 ofthe present invention and hence they are free of cracks and excellent incoating film adhesion. The results suggest that it is necessary tocontrol the composition and the shape of the oxides that separate out onthe surface of the steel sheet for the steel sheet to have good chemicaltreatability and coating film adhesion.

By contrast, samples in Nos. 29, 30, 31, 33, 39, and 41 do not meet therequirement for the steel sheets (1 to 3) of the present invention andhence they are poor in coating film adhesion. In other words, samples inNos. 29 to 31 do not meet the requirement for [Si]/[Mn] ratio and hencethey do not give oxides having the shape specified in the presentinvention. Moreover, they have many cracks (because they are notproduced under the desired conditions) and they are poor in coating filmadhesion.

Samples in Nos. 33, 39, and 41 are not produced under the desirableconditions and hence they do not have oxides with the shape specified inthe present invention. They have cracks and are poor in coating filmadhesion.

Sample in No. 37 meets the requirements and hence is excellent incoating film adhesion; but it cannot be formed satisfactorily on accountof its poor ductility.

Samples in Nos. 1 to 27 meet the requirement for the steel sheet 3 ofthe present invention (or the requirements for the steel sheets (1 and2) of the present invention) and they also satisfy the equations (2) and(3) and hence they are excellent in chemical treatability, coating filmadhesion (free of cracks), and weldability.

Sample in No. 28 meets the requirements for the steel sheet 3 of thepresent invention; however, the results suggest that the compositionshould satisfy the equations (2) and (3) for the steel sheet to exhibitgood weldability.

Samples in Nos. 1, 29, and 34 gave the extraction replicas whoseelectron micrographs (by observation under a TEM) are shown in FIGS. 4to 6. FIG. 4 indicates that sample in No. 1 has fine Si—Mn complexoxides but does not have oxides composed mainly of Si. FIG. 5 indicatesthat sample in No. 29 is covered with oxides composed mainly of Si. FIG.6 indicates that sample in No. 34 does not have fine Si—Mn complexoxides although it has particulate matter (which is rust).

Samples in Nos. 1, 29, and 34 have the cross section near the surface ofthe steel sheet whose electron micrographs (by observation under an SEM)are shown in FIGS. 7 to 9. FIG. 7 indicates that sample in No. 1 is freefrom cracks. FIG. 8 indicates that sample in No. 29 has cracks, 5 μmdeep. FIG. 9 indicates that sample in No. 34 is free of cracks and henceexcellent in coating film adhesion.

Samples in Nos. 1, 29, and 34 have the surface texture whose electronmicrographs (by observation under an SEM) are shown in FIGS. 10 to 12.FIG. 10 indicates that sample in No. 1 has fine zinc phosphate crystalsfree of interstice. FIG. 11 indicates that sample in No. 29 has smallzinc phosphate crystals with large interstices. FIG. 12 indicates thatsample in No. 34 has large zinc phosphate crystals with largeinterstices.

1. A high-strength cold-rolled steel sheet excellent in coating filmadhesion, which is a DP (Dual Phase) steel sheet of ferrite-temperedmartensite type containing no more than 1 mass % of C (excluding 0 mass%), 0.05 to 2 mass % of Si, and 1 to 5 mass % of Mn, having a tensilestrength no lower than 550 MPa, satisfying the equation (1) below, andbeing characterized by its surface in which there exist Si—Mn complexoxides no larger than 5 μm in diameter of the equivalent circle as manyas 10 or more per 100 μm² and the coverage of oxides composed mainly ofSi on the surface of steel sheet is no more than 10% of surface area.[Si]/[Mn]≦0.4  (1) where [Si] denotes an Si content (in mass %) and [Mn]denotes an Mn content (in mass %).
 2. A high-strength cold-rolled steelsheet excellent in coating film adhesion, which is a DP (Dual Phase)steel sheet of ferrite-tempered martensite type containing no more than1 mass % of C (excluding 0 mass %), no more than 2 mass % of Si(excluding 0 mass %), and 1 to 5 mass % of Mn, having a tensile strengthno lower than 550 MPa, and being characterized by its surface whosecross section does not show cracks with a width no larger than 3 μm anda depth no smaller than 5 μm in arbitrary ten fields of observationunder an SEM with a magnification of
 2000. 3. A high-strengthcold-rolled steel sheet excellent in coating film adhesion, which is aDP (Dual Phase) steel sheet of ferrite-tempered martensite typecontaining no more than 1 mass % of C (excluding 0 mass %), 0.05 to 2mass % of Si, and 1 to 5 mass % of Mn, having a tensile strength nolower than 550 MPa, satisfying the equation (1) below, and meeting thefollowing requirements (I) and (II). (I) In the surface of the steelsheet, there exist Si—Mn complex oxides no larger than 5 μm in diameterof the equivalent circle as many as 10 or more per 100 μm² and thecoverage of oxides composed mainly of Si on the surface of steel sheetis no more than 10% of surface area. (II) The cross section near thesurface of the steel sheet does not show cracks with a width no largerthan 3 μm and a depth no smaller than 5 μm in arbitrary ten fields ofobservation under an SEM with a magnification of 2000.[Si]/[Mn]≦0.4  (1) where [Si] denotes an Si content (in mass %) and [Mn]denotes an Mn content (in mass %).
 4. The high-strength steel sheets asdefined in claim 1, which satisfies the equations (2) and (3) below.[P]+3[S]+1.54[C]<0.25  (2)[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3) where [C], [Si], [Mn], [P], and[S] denote the content (in mass %) of these elements.
 5. Thehigh-strength steel sheets as defined in claim 2, which satisfies theequations (2) and (3) below.[P]+3[S]+1.54[C]<0.25  (2)[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3) where [C], [Si], [Mn], [P], and[S] denote the content (in mass %) of these elements.
 6. Thehigh-strength steel sheets as defined in claim 3, which satisfies theequations (2) and (3) below.[P]+3[S]+1.54[C]<0.25  (2)[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3) where [C], [Si], [Mn], [P], and[S] denote the content (in mass %) of these elements.
 7. Thehigh-strength steel sheets as defined in claim 1, wherein the coverageof oxides composed mainly of Si on the surface of steel sheet is no morethan 5% of surface area.
 8. The high-strength steel sheets as defined inclaim 3, wherein the coverage of oxides composed mainly of Si on thesurface of steel sheet is no more than 5% of surface area.